Uranium in the Near-shore Aquatic Food Chain: Studies on Periphyton and Asian Clams

Abstract

SummaryThe benthic aquatic organisms that are found in the near-shore environment of the Columbia River are the first biological receptors that can be exposed to groundwater contaminants coming from the U.S. Department of Energy's Hanford Site. These benthic organisms are attached to the river substrate (e.g., periphyton) or move around within the substrate (e.g., clams) but are not found up in the water column (e.g., phytoplankton or fish like bass). The primary contaminant of concern in the former nuclear fuels processing area at the Site, known as the 300 Area, is uranium. Currently, there are no national clean up criteria for uranium and ecological receptors. This report summarizes efforts to characterize biological uptake of uranium in the food chain of the benthic aquatic organisms and provide information to be used in future assessments of uranium and the ecosystem.Uranium, as it enters the Columbia River from the groundwater, is likely to complex with the ions and other water quality characteristics, and the uranium complexes may be associated with the accumulation of uranium in the aquatic biota. The accumulation of uranium in the periphyton community and Asian clams (Corbicula fluminea) was investigated using laboratory studies, field measurements and ecological risk models. The general conclusions as a result of these investigations are listed below, with additional detail provided in relevant sections in the main body of this report. Laboratory Studies• Periphyton accumulates uranium readily. However, at concentrations as high as 100 μg/L uranium, there was no indication of an impact to the community from the presence of uranium based on biomass and chlorophyll content.• Asian clams accumulate uranium readily, but the rate of accumulation is dependent on water concentration.• Accumulation and loss of uranium in the soft tissue of clams changes based on exposure history. In pulsed-exposure studies where the accumulation was evaluated based on short-and long-term pulse of uranium, the concentration in the clam's soft tissues changed based on how long the tissues were exposed to uranium. Field Measurements• Asian clams represent one of the most common aquatic species collected in the Hanford Reach of the Columbia River. Periphyton has only been collected by a few programs.• Environmental media (e.g., river water and sediment) have been collected frequently, and offer a basis for evaluation in ecological risk assessments. Ecological Risk Modeling• Ecological Contaminant Exposure Model (ECEM) has been developed for the evaluation of risk to the aquatic food chain of the Hanford Reach of the Columbia River.• Parameters for evaluating risk using ECEM were assessed using values from literature sources, as well as calculations from laboratory studies and field measurements. The range and distribution of biological concentration factors (BCFs) from these three sources varied in range and distribution. Results indicated that additional improvements to ecological modeling results would benefit from further refin...